The present invention relates to an installation for storing and recovering heat energy, particularly for a solar power station.
A solar power station must be designed so as to take into account the fluctuating and even intermittent nature of the heat source, since it concerns the sun. Moreover, it is desirable for the installation which uses the heat energy, e.g. an installation converting heat energy into electric energy, to produce this secondary energy even outside periods of sunshine, especially if it is a relatively isolated installation not able to receive a compensating primary energy in any other form, at night or during periods of weak sunshine.
In other words, the production of energy must be able to adapt itself to the demand. To reach this result, it is advisable to store the energy produced and not consumed, when the primary heat energy production is greater than the demand for converted energy, and to take it out of store when the demand for converted energy is on the contrary greater than the primary heat energy production.
The aim of the present invention is essentially to overcome this problem and, to do this, an installation of the type mentioned above is characterized in that it comprises: a first exchanger, associated with a heat source, in which a circulating heat-carrying fluid or thermofluid may undergo a temperature increase; a second exchanger in which said thermofluid may yield heat to a user unit; a first circuit connecting the output of the first exchanger to the input of the second; a second circuit connecting the output of the second exchanger to the input of the first; a storage reservoir containing a material able to store the heat, connected to both the first and to the second circuit; and distributing means adapted to effect automatically a specific distribution, on the one hand of the thermofluid coming from the first exchanger between the second exchanger and the storage reservoir, on the other hand of the thermofluid coming from the second exchanger between the first exchanger and said reservoir.
By "first exchanger" is meant above one or more boilers or heating coils in which flows any heat-carrying fluid, e.g. a thermofluid known commercially as Gilotherm, and on which heating coils the sun's rays may be conentrated by a system of mirrors (heliostats).
As for the second exchanger, it will serve for example for transferring the heat from the heat-carrying fluid to the liquid of a boiler for the production of steam capable of supplying any steam thermodynamic conversion machine whatever (A piston engine, a screw engine, or a turbine) driving for example an electric generator. Steam engines may be preferred, particularly to turbines, particularly in the case of small and medium sized powers, for reasons of efficiency and flexibility of use.
The arrangement and operation of said distributing means will be described with more detail later on but we can already note their general organization as for the flow of thermofluid coming from the first exchanger, i.e. as the case may be, from boilers associated with the heliostats, these distributing means will direct to the user unit a smaller quantity of the thermofluid (and so a larger quantity towards the storage reservoir), the lower the demand is for secondary energy compared with the amount of primary energy collected in the form of heat, and conversely, while assuming that the amount of primary energy collected in the form of heat is greater, at the moment considered, than the energy demanded (heat storage); regarding furthermore the flow of thermofluid supplying the second exchanger, i.e. that in which the heat of the thermofluid is transferred, e.g. to the water of a boiler, said distributing means will take from the storage reservoir a quantity thereof the greater (and so a smaller quantity from the first exchanger), the higher the demand for secondary energy with respect to the amount of energy produced in heat form, and conversely, assuming that the energy produced in the form of heat is less, at the moment considered, than the energy demanded (heat destorage).
These distributing means will also be arranged, correlatively, to take from the storage reservoir, so as to supply it to the input of the first exchanger, a quantity of cold thermofluid equal to that of the hot thermofluid which is taken at the output of the first exchanger to be directed toward the reservoir (case of the above-mentioned storage), and to return to the storage reservoir from the output of the second exchanger, a quantity of cold thermofluid equal to that of the hot thermofluid which has been taken from said reservoir to supply said second exchanger (case of the above-mentioned destorage), all this so that of course the flows are balanced.
We can also add here that the distributing means will also naturally be arranged to ensure an adequate distribution of the thermofluid in the following particularly or boundary cases: (1) when the amount of secondary energy demanded corresponds exactly to the primary heat energy produced, the distributing means are arranged so that, in this case, no thermofluid enters the storage reservoir or or leaves therefrom (storage out of circuit), (2) when the secondary energy demanded is zero, the distributing means are arranged so that, in this case, all the thermofluid coming from the first exchanger flows into the storage reservoir. (3) when the heat energy produced is zero, the distributor means are arranged so that, in this case, all the thermofluid supplying the second exchanger is taken from the storage reservoir. These cases of operation will also be described with more detail herebelow.
As regards now the material capable of storing the heat, so capable of storing the energy in heat form, it may be formed by salts melting easily at a relatively low temperature, e.g. soda (NaOH), sodium nitrate (NaNO.sub.3), potassium nitrate (KNO.sub.3) and similar, the melting temperatures of the above three materials being respectively about 320.degree. C., 300.degree. C. and 280.degree. C.; soda may be preferred since its cost price is lower.
These examples are however not limitative, it being understood that the invention may also use heat storage materials with a lower melting temperature allowing the use of special low temperature thermodynamic cycles or else heat storage materials having a higher melting temperture.
Thus, during periods of storage, the easily melting material is borne by the heat carrying fluid or thermofluid at a temperature greater than its melting point, which allows it to accumulate tangible heat and latent heat of fusion.
The advantage of using such materials resides in large specific storage capacities, in the constancy of the temperature at which the heat is restored (it is practically the temperature of change of state), and in the smallness of the heat losses, the energy stored being almost entirely restored if the heat insulation of the installation is suitable.
Furthermore, it should be noted that the fact of storing the energy in heat form, i.e. before conversion, enables the power of the conversion unit to be reduced and so its cost price, which generally forms a large part of the investment (1/3 to 1/4 of the total price).
However, most of the storage materials which may be considered, examples of which have been given above, have the disadvantage of being poor heat conductors in the solid state, which results in mediocre heat transfers during de-storing, and so another problem of exchange surfaces, specific to the installations of the above-described kind.
The invention has therefore as an aim to overcome this problem also and to this end an installation such as described above may be further characterized in that it comprises means for streaming said thermofluid over the walls of cointainers enclosing said heat storage material, disposed in said storage reservoir, and in that, for this, said storage reservoir extends essentially vertically. Said material for storing the heat, formed particularly from an easily melting substance or similar, contained in said storage reservoir, is distributed over a set of containers whose individual volume is small compared with the total volume of said material. These containers are superposed in said reservoir, over substantially the whole of its height, so that spaces are provided therebetween, to allow free flows of said thermofluid to pass, from the upper part to the lower part of said reservoir.
Thus, it is essentially by streaming the thermofluid over the walls of said containers that the necessary heat exchanges take place between this fluid and the material which stores the heat, enclosed in the containers, whether in the storage phase, in which the hot thermofluid yields heat to said materials, or in the de-storing phase, during which cold thermofluid receives heat, given up by the material.
The optimum dimensions of the containers in question will be determined for example from a mathematically designed pattern, particularly so that practically all the material is melted in each container during a prolonged storage phase, and is practically completely solidified therein during a prolonged de-storage phase.
Said containers may be formed for example by boxes or cans, of the kind used for canned foods or similar, particularly cylindrical, loosely or methodically stacked in said reservoir. Care will however be taken that there are no preferential passages for flows of thermofluid between the cans, which would reduce the heat storage capacity of the assembly.
Many other arrangements could however be provided for containing the storage material, e.g. superposed layers of spaced horizontal tubes, disposed in an alternating arrangement in a storage reservoir having a rectangular or square base, etc... or else cylindrical cans aligned so as to form in the aggregate horizontal tubes, these tubes being spread out in superposed layers and disposed alternately. However, it will certainly be advantageous to provide in all cases for the total volume of the containers to be equal at least to about a half, or substantially more than a half, of that of the storage reservoir.
Thus, to take an example, if the weight of the heat storage material is 60 tons, which corresponds to about 30m.sup.3 of non-conditioned crude soda, the soda containers will be disposed in a storage reservoir of about 60m.sup.3, e.g. with a base of 10m.sup.2 and a weight of 6m, or in a storage reservoir of less volume. This ratio of 1/2 is however given particularly by way of indication and it is evident that it could vary to a large extent without the principle of the invention being modified.
In any case, the fact of providing a heat storage material divided, i.e. spread out in a large number of relatively small containers, will allow the exchange surfaces offered to the streaming of the thermofluid to be effectively increased, for a given mass of said material, and so the heat storage capacity thereof to be better used.
Another problem posed by using an installation of the type described at the beginning and conforming to the invention resides in the fact that the thermofluid taken from the storage reservoir will have to have a temperature compatible with its destination. Thus, it was explained above that thermofluid could be taken from the storage reservoir for forming a supplementary, or make-up, supply with respect to the flow of thermofluid coming from the first exchanger (heat source), to supply the second exchanger (user unit), during a period of de-storage; it will be evident that this thermofluid will have to have a sufficiently high temperature.
It was also explained above that thermofluid could be taken from the storage reservoir for forming a supplementary, or make-up, supply with respect to the thermofluid coming from the second exchanger, for supplying the first exchanger, during a period of storage; it will also be evident that this thermofluid will have to have a sufficiently low temperature.
Another advantageous feature of an installation in accordance with the invention allows this problem also to be surmounted and it consists essentially in dividing the storage reservoir into compartments. More exactly, an installation in accordance with the invention may be characterised in that said storage reservoir comprises several superposed compartments or levels, each of which is provided with a thermofluid reserve collecting the thermofluid which has streamed over the containers of the compartment considered and from which, on the one hand, the thermofluid may be taken by means of an outlet duct, to be directed either to the first exchanger, or to the second, and on the other hand this thermofluid may flow, particularly by overflowing towards the level situated immediately below the level considered, while streaming over the containers of heat storage material of said level located below.
Thus, to each reserve there corresponds a temperature of the thermofluid which comes therefrom, and all that is required is to choose by an adequate procedure the reserve from which it will be extracted, and depending on its destination, e.g. by an intermittent sequential searching of the temperatures of the thermofluid coming from the different reserves, and stopping on the reserve which contains the thermofluid having the correct temperature. The means for doing this will be described in more detail hereafter.
According to yet another characteristic of the invention, it could be arranged for a storage reservoir of circular section that the reserve for each level is annular and surrounds, substantially over the whole of its height, the storage compartment of the level located immediately below, which contains a part of said heat storage material containers.
A variation could also be provided in which, for a storage reservoir having a square or rectangular section, said reserve of each level is formed by two gutters disposed on each side of the storage compartment of the level located immediately below, which contains a part of said heat storage material containers and extends substantially over the same height.
Said part of said heat storage material containers may comprise the same number of containers, having the said individual volume, for each storage compartment of the storage reservoir. Thus, if this reservoir comprises n storage compartments (n levels), each storage compartment may contain the n.sup.th part of the total number of containers.
In practice, it may furthermore be arranged that the bottom of each storage compartment is formed from a grid or grating, or from a perforated metal sheet or the like, capable of holding the heat storage material containers of the level considered and allowing free flow of the thermofluid streaming over said containers towards the reserve of said considered level.
According to another practical arrangement, it may further be provided that below said grid or or the like of each level there is disposed a conical deflector or a deflector in the shape of a dual-pitch roof, for directing the thermofluid which has streamed over the containers of the storage compartment of the level considered towards the reserve of said level.
Also advantageously, the edges of said deflector are formed in the shape of a funnel extended downwardly by an inlet pipe emerging adjacent the bottom of the reserve of the level considered.
It is furthermore advantageous for the outlet duct(s) which enable thermofluid to be taken from the reserve of one level to direct it either to the first exchanger or to the second, to emerge in said reserve essentially adjacent to and at the same level as the inlet ducts.
With this arrangement, it is ensured that the point where thermofluid is taken from one reserve is situated adjacent to and at the same level as the point to which it is brought into this reserve, which gets over the problems of stratification of the temperature in the reserve, which may arise following changes of state in the storage compartment of the immediately higher level. It is also ensured by this means that the temperature of the thermofluid, measured in an outlet pipe, an essential parameter for determining whether this thermolfluid can if necessary be sent to the first exchanger or to the second, is indeed that of the thermofluid after streaming over the containers of the level considered. This precaution will avoid anarchic operation of the automatic system of selection of the outlet ducts (described below).
According to another important arrangement of the invention, it is provided that above the storage compartment of each level of the storage reservoir there is disposed a horizontal distributor formed from a perforated plate or from spaced gutters situated in the same horizontal plane, this distributor collecting the thermofluid overflowing from the reserve of the storage compartment of the level situated immediately above.
It is important that said distributor is perfectly horizontal, to ensure a perfectly homogeneous distribution of the thermofluid above the heat storage material containers, particularly to avoid any preferential streaming paths of the fluid over the containers. The thermofluid will thus be able to be at all times in contact with the whole of the surface of the exchange walls of said containers, which will ensure maximum heat exchanges between this fluid and the divided heat storage material. It is also evident that to the same end the perforations or spaces formed between the gutters have equal sections and are perfectly uniformly spread out over the whole surface of said distributor.
According to yet another feature of the invention, it is provided that the volume of each reserve of thermofluid of the storage reservoir is greater than the volume of the reserve located immediately above it.
More exactly, it is arranged that the volume of a reserve of thermofluid of one level is at least equal to the sum of the volume of the thermofluid reserve of the level situated immediately above and of the volume of the thermofluid streaming into the storage compartment of the level considered.
Thus, if the levels of the storage reservoir are numbered from 1 to n, from top to bottom, the volume of the thermofluid reserve of the first level will be at least equal to v, that of the reserve of the second level will be at least equal to 2v, and so on, and the volume of the lowest reserve of thermofluid will be at least equal to nv, v representing the volume of the thermofluid streaming over the walls of the heat storage material containers of one level.
This arrangement takes into account the time which elapses between the moment when the thermofluid is introduced in the upper part of the storage reservoir and the moment when this same thermofluid reaches its lower part. The reserves thus form efficient buffers for avoiding any delay in taking fluid, or any no load pumping.
The need for providing buffer reserves of sufficient volume and increasing in volume from top to bottom can be understood by assuming that the hot thermofluid is taken from the reserve of the first level to form a supplementary supply for the second exchanger. The cooled thermofluid coming from the second exchanger will return only partially to the first exchanger, the complementary part returning to the upper part of the storage reservoir to balance the flows. But this thermofluid returning to upper part of the storage reservoir will obviously not be able to reach immediately the reserve of the first level, since it will first of all have had to flow over the walls of the containers of this first level. This is why the volume of the reserve of the first level will have to be at least equal to v, so that the return of the thermofluid taken from this reserve can be waited for without no-load pumping.
Of course, if a supplementary reserve of volume v' is provided above the upper level (the advantage of such a reserve will be seen herebelow), similar reasoning shows that the reserve of thermofluid of the first level will have to have a volume at least equal to v+v', that that of the second level will have to have a volume at least equal to 2v+v' and so on, the volume of the lowest reserve then being equal to v'+nv.
It was explained above that in the case where the thermofluid had to be taken from the storage reservoir, it had to be so at a temperature which was best adapted to its destination: a relatively low temperature if it is suitable to send it back to the first exchanger (boiler or heating coils heated by the heliostats), failing which it might be brought in this exchanger up to a temperature greater than its operating temperature limit (about 350.degree. C. for the Gilotherm); and a relatively high temperature if it is suitable to send it back to the second exchanger (user unit), failing which it would be incapable of fulfilling its role, e.g. for causing the evaporation of the water in the boiler of the user unit.
In other words, it is advisable at all times to ensure that thermofluid is automatically taken from the reserve which it leaves at the correct temperature. To do this, an installation in accordance with the invention may be further characterised in that the outlet ducts, each of which comes from a given reserve, and which are meant to supply the first exchanger, are connected respectively to the inputs of a first rotary valve whose output may be connected to said first exchanger, said valve being controlled by a motor slaved to a regulator-comparator receiving on the one hand a signal representative of a first control temperature, on the other hand a signal representative of the temperature of the thermofluid at the output of said valve.
Thus, the motor will automatically stop the valve when the output thereof is connected to an outlet duct of the storage reservoir, in which the thermofluid has a temperature less than or equal to the control temperature. It may moreover be arranged that the regulator-comparator does not take the temperature of the thermofluid into account until after stabilization thereof (it is sufficient to derive the signal and not to take its value into account until this derived signal is zero or very low), this is to avoid any risk of uneven operation--or pumping--of the regulation system.
There may also be provided intermediate reserves for ensuring the permanence of the supply of the first exchanger during the time that the rotary valve takes to pass from one outlet duct to the next, and to avoid any risk of cavitation.
According to yet another feature, it may also be arranged that the motor is coupled to said first rotary valve so that said search of said outlet ducts takes place while passing from one outlet duct connected to one reserve to the outlet duct connected to the reserve immediately above, and so on. In this case, it is in fact by beginning the search of the reserves from the bottom that the one in which the thermofluid is sufficiently cold to be sent to the first exchanger will be the most rapidly found.
Similarly, it may also be arranged that the outlet ducts, each one of which comes from a given reserve and which are intended to supply the second exchanger, are connected respectively to the inputs of a second rotary valve whose output may be connected to said second exchanger, said valve being controlled by a motor slaved to a regulator-comparator receiving on the one hand a signal representative of a second control temperature, on the other hand a signal representative of the temperature of the thermofluid at the outlet of said valve.
In the same way, it will thus be achieved that the motor (second motor) will automatically stop this second valve when the outlet thereof is connected to an outlet duct of the storage reservoir in which the thermofluid has a temperature greater than or equal to the second control temperature. Of course the same supplementary arrangements as those indicated above in relation to the first rotary valve may also be provided.
Advantageously also, it will be arranged that the motor is coupled to said second rotary valve so that the search of said outlet ducts takes place by going from one outlet duct connected to one reserve, to the outlet duct connected to the immediately lower reserve, and so on. In this case, it is by beginning the search of the reserves from the top that the one in which the thermofluid is sufficiently hot to be sent to the second exchanger will be the most rapidly found.
Another essential problem to be resolved, if an entirely automatic operation installation is desired, is to arrange that, on the one hand, the cold thermofluid is automatically taken from the storage reservoir to be sent to the first exchanger during periods of storing heat energy, i.e. when the demand for converted energy is less than the production of heat energy, and that correlatively hot thermofluid, coming from the first exchanger, is sent to said reservoir, and furthermore that hot thermofluid is automatically taken from the storage reservoir to be sent to the second exchanger during a period of de-storage of heat energy, i.e. when the demand for converted energy is greater than the production of heat energy and that correlatively cooled thermofluid, coming from the second exchanger, is sent to said reservoir. It is also a matter of course of allowing automatic operation of the installation in the particular or borderline cases cited above (demand equal to cffer; zero production of heat energy; zero demand for converted energy). These different roles are assumed by the distributing means mentioned at the beginning.
In accordance with the invention, the installation will to this end be characterised in that said distributing means comprise, between the storage reservoir and said first circuit, a first three-way valve, with a double pressure actuatable flap and one way of which, emerging into a chamber intermediate the flaps, communicates with said first circuit and the other ways of which are connected, one to an input communicating with the upper part of the reservoir and the other to a thermofluid outlet, particularly a (second) rotary valve outlet, said first circuit comprising, upstream of said exchanger, a circulating pump slaved to a signal representative of the converted energy demanded. The operation of such a three-way two flap-valve will be explained in more detail with reference to the figures, but here and now the principle thereof can be explained: in the case where the converted energy demanded is greater than the heat energy available(de-storing), a signal, e.g. an electrical signal, will control the circulating pump so that the flow of thermofluid which it sets up in the first circuit increases, which will cause a relative depression upstream of this pump and consequently in the intermediate chamber mentioned. This depression will then control the double flap, so as, on the one hand, to establish communication between the outlet of the rotary valve (here the second valve) and the first circuit to bring a complement of hot thermofluid to the second exchanger and, on the other hand, to cause the communication to cease (if it existed beforehand) between this first circuit and the inlet of the upper part of the storage reservoir.
The case for storing heat energy is treated in an opposite manner, the double flap of the first three-way valve then taking up its other position. Similarly, it may be arranged that said distributing means comprise furthermore, between the storage and said second circuit, a second three-way valve with a double pressure actuatable flap, and one branch of which, emerging into an intermediate chamber between the flaps, communicates with said second circuit, and the other branches of which are connected, one to an inlet communicating with the upper part of the reservoir and the other to a thermal fluid outlet particularly a (first) rotary valve outlet, said second circuit comprising, upstream of the first exchanger, a circulating pump slaved to a signal representative of the heat energy produced by said first exchanger.
It will be readily understood that this second three-way valve operates similarly to the first one.
Thus, to only take up again here a case of operation in correlation with what has been described above (de-storage), to a depression created by this circulating pump in the intermediate chamber of the first three-way valve will correspond a relative over-pressure in the intermediate chamber of the second three-way valve. This valve is of course then arranged so that its double flap takes up a position allowing a connection between the second circuit and the inlet of the upper part of the heat storage reservoir, so that a part of the thermofluid cooled in the second exchanger may be re-heated therein and prohibiting moreover, (if it existed beforehand) communication between this second circuit and the outlet of the rotary valve (here the first valve). Here again, the case of heat storage will be treated in an opposite manner, the double flap of the second three-way valve then taking up its other position. Other types of distributing means could be provided, using for example a subsidiary reservoir with two insulated compartments disposed at the upper part of the storage reservoir. Such a variation will only be described with reference to the figures.